Fluoropolymers have found wide utility in a vast array of applications. However, they are often beset by processing difficulties. Specific difficulties include surface roughness such as melt fracture and other problems such as die drooling.
Higher processing temperatures can reduce some of these problems, but may involve other problems. One such problem is the molecular weight degradation of the fluoropolymer. This can result in reduced physical properties, can contribute to the die drool, and the corrosive by-products can lead to premature wear of the processing equipment.
Another alternative to reduce the surface roughness of extrudates is to reduce the processing rate. This increases the residence time of the fluoropolymer in an extruder, which also contributes to degradation. Reducing production rates is also economically undesirable.
Decreasing the molecular weight of the input fluoropolymer can allow for limited improvements in output, but this also decreases the mechanical properties of the polymer. Such a mechanical property detriment may then be partially offset by the addition of costly comonomers, but this modification can add production complications and detract from other physical properties.
Another approach toward reducing surface defects in fluoropolymers has been to create a mixture of several fluoropolymers having similar composition yet of significantly different molecular weights in attempt to balance the polymer properties with the processing parameters. In theory, a lower molecular weight portion allows for higher output rate with the blend, while a higher molecular weight portion improves the mechanical properties of the blend. This compromise achieves limited success and increases the complexity required to produce such a material.
Yet another approach involved adding a polyolefin to specific fluoropolymers. However, the temperatures necessary for processing fluoropolymers are usually too high for this approach. In addition, such a material can negatively affect properties of the fluoropolymer, such as color, permeation rate, and chemical resistance.
JP 60-23701 describes a blend of a fluorinated elastomer and a copolymer of tetrafluoroethylene and hexafluoropropene (FEP) to achieve heat stress-crack resistance. U.S. Pat. No. 5,051,479 describes a melt-processable thermoplastic consisting essentially of a blend of a fluoropolymer and an elastomeric tetrafluoroethylene-perfluoro(alkyl vinyl) ether copolymer.